Various methods and systems are known in the art for tracking the coordinates of objects involved in medical procedures. Some of these systems use magnetic field measurements. For example, U.S. Pat. Nos. 5,391,199 and 5,443,489, whose disclosures are incorporated herein by reference, describe systems in which the coordinates of an intrabody probe are determined using one or more field transducers. Such systems are used for generating location information regarding a medical probe or catheter. A sensor, such as a coil, is placed in the probe and generates signals in response to externally-applied magnetic fields. The magnetic fields are generated by magnetic field transducers, such as radiator coils, fixed to an external reference frame in known, mutually-spaced locations.
Additional methods and systems that relate to magnetic position tracking are also described, for example, in PCT Patent Publication WO 96/05768, U.S. Pat. Nos. 6,690,963, 6,239,724, 6,618,612 and 6,332,089, and U.S. Patent Application Publications 2002/0065455 A1, 2003/0120150 A1 and 2004/0068178 A1, whose disclosures are all incorporated herein by reference. These publications describe methods and systems that track the position of intrabody objects such as cardiac catheters, orthopedic implants and medical tools used in different medical procedures.
It is well known in the art that the presence of medical and surgical tools comprising metallic, ferromagnetic and/or paramagnetic material within the magnetic field of a magnetic position tracking system often distorts the system measurements. In some cases the distortion is caused by eddy currents that are induced in such objects by the system's magnetic field. In other cases the interfering object distorts the system's magnetic field itself.
Various methods and systems have been described in the art for performing position tracking in the presence of such interference. For example, U.S. Pat. No. 6,147,480, whose disclosure is incorporated herein by reference, describes a method in which the signals induced in the tracked object are first detected in the absence of any articles that could cause parasitic signal components. Baseline phases of the signals are determined. When an article that generates parasitic magnetic fields is introduced into the vicinity of the tracked object, the phase shift of the induced signals due to the parasitic components is detected. The measured phase shifts are used to indicate that the position of the object may be inaccurate. The phase shifts are also used for analyzing the signals so as to remove at least a portion of the parasitic signal components.
Another application is described in U.S. Pat. No. 5,767,669, whose disclosure is incorporated herein by reference. The inventors describe a system for determining the position of remote sensors using pulsed magnetic fields. The fields are sensed by a remote sensor comprising a plurality of passive field sensing elements. Eddy current distortions are sensed separately and subtracted by the system. The system measures the effect of metallic objects present in the environment and dynamically adjusts the measured values accordingly. The sensed magnetic fields are used in order to calculate the position and orientation of the remote object.
Reduction of eddy currents induced in objects is sometimes implemented by constructing the objects from laminated layers of thin, metallic plates or from powdered-iron. For example, U.S. Pat. No. 6,178,353, whose disclosure is incorporated herein by reference, describes an implantable medical device that utilizes laminated, sectionalized or particle-based structures, so as to reduce the electrical energy absorbed by the implant device when in use. The inventors claim that this construction makes the implant device immune to being damaged by magnetic resonance imaging (MRI).
In some applications, the laminated plates are also slotted, in order to reduce eddy currents within the layers. For example, U.S. Pat. No. 5,555,251, whose disclosure is incorporated herein by reference, describes a method for constructing parts of a magnetic resonance medical imager using laminated layers cut from transformer sheet material. Each layer is electrically insulated from adjacent layers and surfaces by enamel or fixing glue. To reduce eddy currents in these layers, narrow, radially oriented slots are cut in the layers before lamination. The slots are oriented in the adjacent layers so as not to coincide. The inventors claim that this construction improves the conduction of the magnetic flux in the imaging volume.
In some applications, the material composition of medical tools is chosen so as to reduce the magnetic field distortion caused by the tool. For example, U.S. Pat. No. 6,258,071, whose disclosure is incorporated herein by reference, describes a medical needle made of non-metallic non-magnetic materials, such that medical interventional procedures requiring needle access to people, animals or isolated tissues can be performed in a Magnetic Resonance Imaging (MRI) scanner without significant artifact or image distortion. The dimensions of the needle are adaptable to the task required.
As another example, U.S. Pat. No. 5,738,632, whose disclosure is incorporated herein by reference, describes a device for use in combination with an MRI apparatus. The distal end portion of the device has a magnetic permeability of such a value that diagnosis and treatment are not influenced by magnetic resonance image distortions due to a disturbance to the uniformity of the static magnetic field generated by the MRI apparatus.